1. Field of the Invention
The present invention relates to a multisource type X-ray CT apparatus for use in three-dimensional image diagnosis.
2. Description of the Related Art
For a high-speed X-ray CT scanner, by using an electron beam control system for electrically turning X-ray generation on and off, the scan time of a certain X-ray CT scanner has heretofore been remarkably accelerated ({fraction (1/60)} to {fraction (1/2000)} second), and tomography of a measurement object has been performed. This high-speed X-ray CT scanner is proposed as an image diagnosis apparatus (multisource type X-ray CT apparatus) including a large number of X-ray sources, for example, in Jpn. Pat. Appln. KOKAI Publication Nos. 10-295682 and 10-075944.
As shown in FIG. 1, a conventional multisource type X-ray CT apparatus includes: a plurality of detectors 102 arranged and fixed at equal pitch intervals in a concentric circle which surrounds an image pickup area 104; a vacuum chamber 105 which is disposed further outside so as to surround a group of detectors 102 and which has a double tube structure; a plurality of X-ray generation units 101 to 132 contained in the vacuum chamber 105; and an X-ray generation control apparatus (not shown). The X-ray generation units 101 to 132 include 32 3-pole vacuum tubes densely arranged in the concentric circle, and each unit irradiates a subject (not shown) disposed in the image pickup area 104 with fan-beam X-rays (fan beams) 3.
The X-ray generation control apparatus includes 32 pulse generation control ports which have a one-to-one correspondence with pulse generators disposed for the respective X-ray generation units 101 to 132, selects the X-ray generation unit optimum for image pickup based on predetermined input data, and controls the on/off switching of a power supply circuit at a high speed so that the fan-beam X-rays 3 (spread angle 2xcex1) are emitted only from the selected X-ray generation unit.
The fan-beam X-rays 3 emitted from the X-ray generation unit are passed through the subject (not shown) of the image pickup area 104 and one incident upon the detectors 102 on a back side. Thereby a transmitted X-ray amount is detected. Detection signals are sent to a data storage apparatus from the detectors 102, stored in the data storage apparatus, and processed by a data processing apparatus. Data obtained by processing the signals is reproduced as an X-ray tomography image on a display.
However, in the conventional apparatus, installation space is restricted by spatial arrangement in relation to the subject, sizes of the vacuum chamber, X-ray generation unit, and detector are limited, and therefore the number of X-ray generation units which can be arranged is limited. Therefore, a large number of X-ray generation units cannot densely be arranged, space resolution of the apparatus cannot be enhanced, and therefore the image reproduced from the transmitted X-ray data is blurred.
Moreover, in each X-ray generation unit of the conventional apparatus, the cathode, anode, and gate (grid electrode) require power supply circuits, and the power source capacity becomes enormous. Particularly when the space resolution is enhanced, the total number of power supply circuits is vast. This causes the problem that not only the manufacturing cost but also the running cost of the power supply circuit increase.
The present invention has been developed to solve the above-described problem, and an object thereof is to provide a multisource type X-ray CT apparatus in which a high space resolution is fulfilled in a limited installation space, a clear image can be obtained, and manufacturing and running costs can be reduced.
According to the present invention, there is provided a multisource type X-ray CT apparatus comprising: a sensor array including a plurality of detection devices densely fixed on a circumference which surrounds a subject in order to detect X-rays transmitted through the subject; a vacuum chamber fixed so as to surround the sensor array coaxially with arrangement of the sensor array; and an X-ray generation unit which is disposed in the vacuum chamber and which emits X-rays toward the subject surrounded by the sensor array.
The X-ray generation unit includes: a circular-arc or linear cathode which is fixed/disposed in the vacuum chamber so as to surround the sensor array coaxially with the arrangement of the sensor array and which emits electron beams by power supply; a circular-arc or linear anode disposed in a position upon which the electron beams emitted from the cathode are incident, and fixed/disposed in the vacuum chamber so as to surround the sensor array coaxially with the arrangement of the sensor array, so that the electron beams are received and the X-rays are emitted; a gate array including a plurality of grid electrodes which are densely fixed between the cathode and anode and which include holes for sucking and passing the electron beams emitted from the cathode; a power source which applies a bias voltage to the grid electrodes of the gate array; and control means for controlling a power supply operation from the power source so as to select the grid electrode suitable for image pickup from the gate array in accordance with an image pickup portion of the subject and to release the bias voltage applied to the selected grid electrode.
In this case, the gate array may include 60 to 240 grid electrodes, further include 150 to 300 grid electrodes, and further include 240 to 500 grid electrodes. With an increase of the number of grid electrodes, resolution of the image pickup portion is enhanced and the image becomes clear. For example, it is possible to obtain an X-ray CT section image of blood vessels such as an artery. On the other hand, when the number of grid electrodes is increased in the gate array, the width per grid electrode is excessively narrowed, and this makes it difficult to form the electron beam passing holes. Moreover, the X-ray generation unit is limited by the ceiling height of the room where the unit is installed. Since the diameter of the unit cannot needlessly be enlarged, the increase of the number of grid electrodes is limited to some degree. Therefore, an upper limit of the number of grid electrodes in the gate array is set to 500.
A method of forming the gate array comprises: laminating a layer of a high-melting metal or alloy on a ceramic ring substrate using a physical or chemical vapor deposition method; and subsequently using a wet or dry etching method to partially remove the layer of the high-melting metal or alloy and form the grid electrodes whose adjacent portions are insulated from one another.
It is preferable to use materials having high pressure resistance and insulating properties, such as silicon nitride (Si3N4), silicon oxide (SiO2), silicon carbide (SiC), alumina (Al2O3), and sialon (SiAlON) in the ceramic ring substrate. Especially, high-purity alumina is suitable as the insulating material having high pressure resistance for the ceramic ring substrate.
A single metal such as tungsten, molybdenum, and tantalum is preferably used in the high-melting metal, and an alloy containing one or two or more of tungsten, molybdenum, and tantalum as a main component is preferably used in the high-melting alloy.
It is preferable to use various CVD methods, an ion plating method, and a sputtering method in the physical or chemical vapor deposition method. Additionally, it is especially preferable to use a plasma CVD method among various CVD methods. Because a metal or alloy layer formed by the plasma CVD method is suitable to remove by the dry etching in which photolithography is used.
Furthermore, the electron beam passing holes are preferably made in the grid electrodes by machining. It is preferable to appropriately select the diameter of each electron beam passing hole in a range of 1 to 5 mm in accordance with the size of the grid electrode.
The cathode may be a combination of a plurality of circular-arc or linear segment electrodes arranged on the same circumference, or may be a single annular electrode. When the cathode includes a plurality of circular-arc or linear segment electrodes, the grid electrodes of the gate array are divided into groups including the same number of electrodes. A control circuit is preferably constituted in which the grid electrodes of each group are allocated to the corresponding segment electrodes. Additionally, metal materials such as W, BaW, NiCr alloy, NiCrCo alloy, and NiCrFe alloy, or nonmetal materials such as LaB6 may be used in a cathode material.
Moreover, similarly as the cathode, the anode may also be a combination of a plurality of circular-arc or linear segment electrodes arranged on the same circumference. The corresponding anode is divided into several segment blocks similarly as the cathode and the operation of each block can be controlled. Additionally, it is general to use tungsten or a tungsten alloy in the anode material.
According to the present invention, there is provided a multisource type X-ray CT apparatus comprising:
a donut-shaped gantry to define a space for diagnosis in which a subject is inserted along an axial center;
an annular vacuum tube disposed along a circumference of a concentric circle centering on the axial center in the gantry;
a cathode array including a plurality of discharge electrodes which are disposed in the vacuum tube and densely fixed over at least a half circumference having a center angle of 180xc2x0 along the circumference of the concentric circle centering on the axial center;
an anode array including a plurality of target electrodes which are disposed opposite to the discharge electrodes in the vacuum tube so as to have one-to-one correspondence with the discharge electrodes and densely fixed over at least the half circumference having the center angle of 180xc2x0 along the circumference of the concentric circle centering on the axial center and which generate X-rays by incident electron beams emitted from the discharge electrodes;
a gate array including a plurality of grid electrodes which are disposed between the cathode array and anode array and which tolerate or limit passage of the electron beams toward the target electrodes from the discharge electrodes by control of an applied voltage;
a first insulating member which insulates the discharge electrodes of the cathode array from the vacuum tube;
a second insulating member which insulates the target electrodes of the anode array from the vacuum tube;
a third insulating member which insulates the grid electrodes of the gate array from the vacuum tube; and
a plurality of X-ray detection portions densely fixed/arranged over at least the half circumference having the center angle of 180xc2x0 along the circumference centering on the axial center so as to have one-to-one correspondence with the target electrodes of the anode array via the subject.
Additionally, the center angle of the arrangement of the X-ray detection portions is set to be larger than the center angle of the arrangement of the anode array or cathode array by an X-ray spread angle 2xcex1. All that X-rays the pass through the subject are detected by the X-ray detection portions, so that much information is obtained.
The above-described first, second, and third insulating members are formed of continuous annular or circular-arc ceramic, and materials such as silicon nitride (Si3N4), silicon oxide (SiO2), silicon carbide (SiC), alumina (Al2O3), and sialon (SiAlON) are preferably used. Particularly, it is preferable to use ceramic materials having high pressure resistance and insulating property, such as high-purity alumina.